Old Irish neuter u-stem nouns.

These nouns are neuter, typically with the following endings:

• in the nominative singular: the bare stem (always ending in an unpalatalized consonant), with nasalization of the following word;
• in the genitive singular: -o or -a (with lowering of the stem vowel where possible), with aspiration/gemination of the following word and sometimes with palatalization of the final stem consonant;
• in the nominative plural: either (a) the bare stem with lenition of the following word; or (b) -a (used especially with an indefinite meaning), with lowering of the stem vowel where possible and sometimes with palatalization of the final stem consonant.

The stem classes are named from the perspective of Proto-Celtic and may not still be visible in Old Irish inflections.